Desulphurization of odorous gases of a pulp mill

ABSTRACT

The invention relates to a method and an arrangement for the removal of concentrated odorous gases of a pulp mill. According to the method, the concentrated odorous gases containing sulphur compounds are collected from the odorous gases of the sulphite pulp mill and combusted with under air to oxidize the sulphur compounds, so that at least an essential part of the sulphur compounds is oxidized into elemental sulphur. The elemental sulphur is recovered in a liquid or solid form. It can be returned to the process or used in other processes.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is entitled to the benefit of and incorporates byreference essential subject matter disclosed in International PatentApplication No. PCT/FI2004/000581 filed on Oct. 4, 2004 and FinnishPatent Application No. 20031438 filed Oct. 3, 2003.

FIELD OF THE INVENTION

The present invention relates to a method for the desulphurization ofthe circulation of chemicals in a sulphate pulp mill.

BACKGROUND OF THE INVENTION

According to a method of this kind, concentrated odorous gases of theprocess streams in sulphate processes, which gases contain sulphurcompounds, are collected and these odorous gases are combusted in orderto oxidize the sulphur compounds.

The circulations in modern pulp mills are closed so that, althoughnon-sulphurous chemicals are substituted for sulphur-containing freshchemicals, the circulation must be desulphurized to preventuncontrollable sulphur emissions. All modern sulphate pulp mills burnconcentrated odorous gases either in soda recovery boilers, lime sludgereburning kilns or in separate odorous gas boilers or reserve flames.

When the concentrated odorous gases are burnt in the soda recoveryboiler, sulphur is removed either by flue gas emissions (sulphurdioxide) or in the form of pulverized fuel ash. Generally, pulverizedfuel ash is discharged from the system by releasing it into the riverand lake system. Sulphur dioxide-containing flue gas emissions causeacidification of soil, damages to the trees and to the surface materialsof buildings, and other environmental damages. Ash, in turn, containsabundantly wood-based heavy metals; therefore, it is possible that therelease of pulverized fuel ash into the water system will in the futurebe prohibited on environmental grounds. A further considerabledisadvantage related to the removal of pulverized fuel ash is that,along with sulphur, also sodium in a molar ratio of Na/S 2/1 containedin the ash exits, and must then be added to the system.

When concentrated odorous gases are combusted in the lime sludgereburning kiln, the sulphur either exits as flue gas emission (sulphurdioxide and TRS=total reduced sulphur, i.e., so-called smelly sulphurcompounds) or in waste lime/waste lime sludge. In addition to the abovedisadvantages, sulphur also causes running failures (cycles, amongothers) in the lime sludge reburning kiln.

When concentrated odorous gases are burnt in the separate odorous gasboilers, either sodium bisulphite or a dilute sulphur dioxide solutionis obtained. In sodium bisulphite, sodium in a ratio of Na/S=1/1 exitsthe system along with the sulphur.

When burning concentrated odorous gases in the reserve flame, allsulphur escape as air emission in the form of sulphur dioxide.

All desulphurization methods currently used have one or more of thefollowing disadvantages: emissions into the air increase, losses ofsodium increase, whereby the operating expenses grow because of thesubstitution chemical needed, or the end product obtained in thedesulphurization is useless to the mill.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to eliminate problems relatedto known technology and to provide a completely new solution forrecovering sulphur from the process streams of a sulphate process.

The present invention is based on the idea that the sulphur contained inconcentrated odorous gases is recovered as elemental sulphur which canbe returned to the process or which can be employed in other practicalapplications.

Selective oxidation of sulphur compounds into elemental sulphur ispreviously known in the art. Accordingly, U.S. Pat. No. 4,522,746discloses the treating of sulphur-containing gas flow by convertingessentially all sulphur compounds in the gas flow first to hydrogensulphide and oxidation of hydrogen sulphide into elemental sulphur atlow temperature (160-320° C.) in the presence of titanium dioxidecatalyst. FI Patent Specification No. 102250 discloses a catalyst forthe selective oxidation of sulphur compounds into elemental sulphur, amethod for preparing the catalyst, and a method for the selectiveoxidation of sulphur compounds into elemental sulphur. Published FIPatent Application No. 903673, in turn, suggests monolithic catalystsfor the treatment of sulphur compounds-containing gases in the industry.Published FI Patent Application No. 933481 discloses a method and anarrangement for the treatment of black liquor originating from asulphate process for the recovery of energy and chemicals. Hydrogensulphide can be produced from exhaust gas containing sulphur compoundswhen heating the black liquor. Hydrogen sulphide can be used fordifferent purposes, for example it can be oxidized into elementalsulphur.

The published GB Patent Application No. 2117749 describes a method andan apparatus for the combustion of gases containing hydrogen sulphide toform elemental sulphur.

None of the above solutions suggests the treatment of concentratedodorous gases; neither do they contain any suggestions for the recyclingof the sulphur thus obtained.

According to the invention, the odorous gases collected from the processstreams of the sulphate pulp mill are combusted at a low air index sothat at least an essential part of the sulphur compounds is oxidizedinto elemental sulphur, which is removed from the system. This solutioncan be used to remove sulphur from the waste gases of the pulp mill in acontrollable way. The method of oxidation of sulphur compounds used inthe invention is based on the so-called Claus process.

The invention also provides an arrangement for the removal of sulphurfrom the circulation of chemicals of a sulphate pulp mill, comprising acombustion unit for the odorous gases, which is provided with an inlet,which is connected to the source of concentrated odorous gases, and withan outlet, through which the oxidized sulphur compounds of theconcentrated odorous gases can be removed from the device as elementalsulphur. The combustion unit according to the invention comprises atleast one combustion device of the Claus system.

More specifically, according to the invention, the method for thedesulphurization of the circulation of chemicals in a sulphate pulpmill, includes the steps of comprising collecting concentrated odorousgases that contain sulphur compounds and combusting them to oxidize thesulphur compounds, wherein the concentrated odorous gases are combustedat an air index below one in a first combustion unit, using the Clausprocess so that at least an essential part of the sulphur compounds areoxidized into elemental sulphur, nonetheless so that at least 10 molar %of the sulphur compounds taken through the Claus system are taken to becombusted in the combustion device of the second combustion unit, andthe elemental sulphur is recovered in liquid or solid form.

According to the invention, the Claus process is used for thedesulphurization of the circulation of chemicals in a sulphate pulpmill, so that the concentrated odorous gases are combusted withsubstoichiometric amounts of air into elemental sulphur in one or morecombustion units of the Claus system and at least 10 molar % of thesulphur compounds taken through the Claus system are taken to becombusted in the combustion device of the at least one second combustionunit.

The invention provides considerable advantages. Accordingly, the methodcan be used to desulphurize the waste gases of the sulphate pulp mill inparticular. The sulphur of the odorous gases is recovered in a form thatcan be utilized in the same or in other processes.

The methods according to known technology provide no solutions, which,according to the invention, would integrate the method ofdesulphurization into the exhaust system of odorous gases of a sulphatecellulose mill.

To optimize the discharge of elemental sulphur in various temperatureranges, the combustion of odorous gases can be carried out in one ormore steps. This can be carried out so that one or more combustion unitsof the Claus system are arranged sequentially. Various runningconditions can be used therein.

On the other hand, the invention can be used for converting sulphur, forexample, so that it can be exploited by combusting sulphur to sulphurdioxide which can be used in bleaching, and to make sulphuric acid bymeans of processing either for the manufacture of bleaching chemicals orfor peeling pine oil. At the same time, the sodium losses of the millcan be minimized.

In a process configuration, the solution can be placed in connectionwith the existing combustion of concentrated odorous gases, wherebyso-called tail gases can be combusted in the existing equipment, such asin a soda recovery boiler, an odorous gas boiler, a lime sludgereburning kiln or a flame.

In the method according to the invention, it is preferable to use forthe condensation of sulphur, e.g., the condenser water or the boilerwater of the pulp mill.

In the arrangement according to the invention, the burning device of theClaus system, i.e. the reactor, and a corresponding steam boiler isconnected at its inlet to the odorous gas line of the cooking departmentor the evaporation plant of the sulphate process or a correspondingsource of odorous gas, and the concentrated odorous gases obtainedtherefrom can be combusted, in accordance with the principles of theClaus process, with substoichiometric amount of air to oxidize thesulphur compounds. To condensate the oxidation products obtained fromthe combustion device, the arrangement further comprises a condenser,which can be used to condensate the products, the elemental sulphur inparticular, into a liquid or solid form. In order to discharge theseliquid or solid substances, the combustion device is equipped with adischarge assembly that is connected to the condenser. In addition, theunit further comprises an exhaust assembly for gaseous (uncondensed)bodies, which can be used to remove any light oxidation products, aswell as inert and not reacted compounds (TRS and SO₂). As indicated indetail in the following, at least part of the sulphur compounds of theconcentrated odorous gases coming from the Claus combustion areconducted to conventional odorous gas combustion which takes place inthe soda recovery boiler, odorous gas boiler, lime sludge reburning kilnor flame, or in more than one of these devices. Therefore, the outlet ofthe gaseous compounds of the arrangement is preferably connected bymeans of pipe lines to the inlet of the soda recovery boiler or the limesludge reburning kiln, or to a similar post-processing device.

In a process configuration, the solution can be placed in connectionwith the existing combustion process of concentrated odorous gases,whereby the so-called tail gases can be combusted in the existingequipment, such as in a soda recovery boiler or lime sludge reburningkiln or similar equipment, and the required condenser of sulphur on thecooling water side can be integrated into the watercirculations/recovery of heat of the mill. In that case, the equipmentsolution is reduced into a simple and economic form. The solutionaccording to the invention can be applied both to existing and newmills.

As a simple addition, the solution is particularly well suited for millconcepts, wherein the soda recovery boiler is the primary place ofcombustion of concentrated odorous gases.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention is examined more closely by means of adetailed description with reference to the appended drawings.

FIG. 1 shows a flow chart illustrating the collection of odorous gases.

FIG. 2 shows, again with the aid of a flow chart, the processing methodof odorous gases according to prior art.

FIG. 3 shows the recovery of sulphur by means of the Claus process,according to the invention.

FIG. 4 shows the flow chart of an alternative embodiment of theinvention, wherein several burning devices of the Claus system areconnected in parallel.

FIG. 5 shows the recovery of sulphur by means of the Claus process andthe flow chart of the post processing of the gases from the Claussystem.

DETAILED DESCRIPTION OF THE INVENTION

One special feature of the invention is that the so-called Claus processis used to binding sulphur from concentrated odorous gases. Thefollowing reactions take place in the Claus process:H₂S+3/2O₂<−>SO₂+H₂O 1/3   (I)2 H₂S+SO₂<−>3 S+2 H₂O 2/3   (II)

The concentrated odorous gases are combusted with a substoichiometricquantity of air or oxygen, whereby part of the sulphur of theconcentrated odorous gases remains elemental sulphur and part iscombusted to sulphur dioxide. The amount of elemental sulphur remainingis 50 to 90 molecular percent, typically 70 to 85 molecular percent.

The concentrated odorous gases herein refer to the smelly gases of thepulp mill, the concentration of which exceeds the so-called upperexplosion limit. These gases consist of hydrogen sulphide, methylmercaptan, and dimethyl sulphide and dimethyl disulphide, among others.

The reduced sulphur in the concentrated odorous gases is enoughconcentrated, is also sodium-free and has a sufficiently rich thermalcapacity. By burning at a suitable air index, the main part of thesulphur can be oxidized into elemental sulphur, as is the case in theabove conventional Claus process. The sulphur can be condensed anddischarged separately as liquid sulphur, which is an inert chemicalelement with an inherent market value. The “air index” herein refers tothe molecular proportion of oxygen (O₂) in the air to the sulphur (S) inthe substance being treated. The air index is one, when the amount ofoxygen in the air is in a stoichiometric proportion to the amount ofsulphur of the gas to be treated to change all the sulphur into sulphurdioxide. “Substoichiometric amounts of air”, in turn, means that the airindex is smaller than one.

In the present invention, the odorous gases are combusted with under airor short air, typically, at an air index of 0.4-0.9, whereby theadiabatic combustion temperature is about 1400-1800° C. For one gasmixture, the combustion was carried out at an air index of about 0.6,whereby the adiabatic combustion temperature was about 1600° C.

According to one preferred embodiment of the invention the combustion ofconcentrated odorous gases is carried out in adiabatic conditions at thetemperature of 1400-1800° C.

According to the invention, part of the collected odorous gases areconducted to Claus combustion and part of the sulphur compounds from theClaus combustion are conducted to conventional odorous gas combustion,or optionally part of the collected odorous gases are conducted directlyto conventional odorous gas combustion.

More specifically, according to the method of the invention, the flow ofodorous gases collected from the source of concentrated odorous gases isconducted to combustion in a first combustion unit, using the Clausprocess, through appropriate pipe lines, and the flow of gases from thefirst combustion unit through appropriate pipe lines to a secondcombustion unit, such as the odorous gas boiler, soda recovery boiler,lime sludge reburning kiln or flame, or to more than one of thesedevices. Alternatively, part of the flow of concentrated odorous gasescan be conducted directly past the first combustion unit and into thesecond combustion unit.

According to the invention, at least 10 molar percent of the collectedodorous gases are conducted to combustion in the first combustion unitusing the Claus process. 30-90 molar %, typically 50-90 molar %, arepreferably conducted through the Claus process. The rest of thecollected odorous gases, 10-90 molar %, can be conducted directly tocombustion in the existing apparatuses.

The concentrated odorous gases are combusted in the first combustionunit with substoichimetric amounts of air. The sulphur that has oxidizedinto elemental sulphur is condensed with the condenser and discharged ina molten form through the bottom of the odorous gas combustion unit. Theelemental sulphur in liquid form is conducted into the collector andtaken further to a means of transport. When cooling, liquid sulphursolidifies; however, short storage for reuse in particular should beimplemented in liquid form.

When necessary, the exhaust gases or tail gases are conducted into thesecond combustion unit of the Claus system to maximize thedesulphurization. If required, the running conditions used in the lattercombustion device may differ from those in the first combustion device.

The exhaust or tail gases, such as sulphur dioxide and TRS, which comefrom the Claus system, are lead into at least one combustion device ofthe second combustion unit, such as the odorous gas boiler, sodarecovery boiler, lime sludge reburning kiln or flame, to be disposed of.In this method, the Claus system is not intended for a complete recoveryof sulphur as elemental sulphur; on the contrary, at least 10 molecularpercent of the sulphur compounds taken through the Claus system aretaken to be combusted in the combustion device of the second combustionunit. Typically, at least 10 molecular percent, about 50 molar % at amaximum, preferably about 15 to 30 molar % of the sulphur compounds ofthe concentrated odorous gases coming from the Claus combustion are leadto combustion in conventional combustion systems, such as soda recoveryboiler or lime sludge reburning kiln.

The method according to the invention functions without catalysts. Whennecessary, the Claus system uses technology, such as catalysts, whichwas developed in connection with the Claus combustion of hydrogensulphide. Examples of suitable catalysts are cited in the published FIPatent Application No. 904949 or FI Patent Specification No. 102250.

The elemental sulphur obtained according to the method can be stored ina liquid or solid form and further transported in a liquid or solidform, for example, to a sulphuric-acid plant or another pulp mill. Itcan also be recycled into the process, e.g., by suitably adding to whiteliquor (the manufacture of a polysulphide solution) or, when necessary,by feeding upstream into the soda recovery boiler by means of acombination burner or in some other way to adjust the sulphidity. As itis easy to store and transport, the recovery of sulphur in a liquid orsolid form is advantageous.

According to an alternative solution, the odorous gases can be combustedin a Claus reactor with the aid of oxygen instead of air.

According to a second alternative solution, the elemental sulphur canfurther be combusted to sulphur dioxide or processed to form sulphuricacid. Also in that case, however, the elemental sulphur can first berecovered by means of the combustion unit of the Claus process.

According to a third alternative solution of the invention, the amountof concentrated odorous gases is increased, for example, by means ofthermal treatment of black liquor.

In the following, the method according to the invention is examined withreference to the appended figures.

FIG. 1 shows a flow diagram of the collection of concentrated odorousgases into the collectors of a cooking department 1 or an evaporatorplant 2 from known sources of concentrated odorous gases eitherseparately or in combinations thereof, using known methods.

FIG. 2 shows a flow diagram of a prior art processing method of odorousgases, wherein the concentrated odorous gases are lead into an odorousgas boiler 3 (containing a condenser 7), soda recovery boiler 4, limesludge reburning kiln 5 or flame 6, or into more than one of them.

FIG. 3 shows the arrangement according to the invention suitable for thecombustion of odorous gases. The arrangement according to the inventioncomprises at least one odorous gas combustion unit 10 (a firstcombustion unit), which a flow of concentrated odorous gases can becollected in and lead into from a source of concentrated odorous gases,and wherein the reduced sulphur contained in the concentrated odorousgases can be combusted. The combustion unit comprises at least onecombustion device 10 of the Claus system, and a condenser 11 connectedthereto, and an air feeding assembly 18. The combustion unit is providedwith a feeding assembly 14, which is connected to the source ofconcentrated odorous gases, and an exhaust assembly 15, through whichthe oxidized and condensed sulphur compounds of the concentrated odorousgases can be discharged from the device as elemental sulphur in a liquidor solid form. The elemental sulphur can be lead into a collector 12 andfurther to a transporter 13. The combustion unit also comprises anexhaust assembly 16, through which the gaseous, uncondensed sulphurcompounds coming from the first combustion unit can be lead into thesecond combustion unit to be combusted. Furthermore, the feedingassembly 14 of the first combustion unit can be connected to adistribution piece 17 so that part of the concentrated odorous gases canbe lead through the distribution piece and past the first combustionunit directly into the second combustion unit to be combusted.

The second combustion unit comprises at least one combustion device,such as an odorous gas boiler, soda recovery boiler, lime sludgereburning kiln and/or flame.

FIG. 4 shows an arrangement, wherein the first combustion unit comprisesat least two combustion devices 20, 21 of the Claus system, connected inparallel, and condensers 22, 23 connected thereto, and air feedingnozzles 33, 34. The combustion unit is provided with a feed nozzle 26,which is connected to the source of concentrated odorous gases, and withan exhaust nozzle 27, through which the oxidized and condensed sulphurcompounds of the concentrated odorous gases can be discharged from thedevice as elemental sulphur in a liquid or solid form. The elementalsulphur can be lead into a collector 24 and further to a transporter 25.The combustion unit also comprises an exhaust assembly 28, through whichthe gaseous, uncondensed sulphur compounds coming from the firstcombustion unit can be lead into the second combustion device 21 of theClaus system through a feeding assembly 29. The latter combustion device21 is provided with an exhaust assembly 32, through which the oxidizedand condensed sulphur compounds can be discharged from the device aselemental sulphur in a liquid or solid form. The elemental sulphur canbe lead into the collector 24 and further to the transporter 25. Thesecond combustion device of the Claus system is further provided with anexhaust assembly 30, through which the gaseous, uncondensed sulphurcompounds coming from the combustion device of the Claus system can belead into the second combustion unit to be combusted.

The feed nozzle 26 of the first combustion unit can be connected to thedistribution piece 31 so that part of the concentrated odorous gases canbe lead through the distribution piece and past the first combustionunit directly into the second combustion unit to be combusted.Alternatively, the exhaust nozzle 28 of the first Claus combustiondevice of the Claus system can also be connected to the distributionpiece, through which part of the gas stream from the first Clauscombustion device can be lead directly into the second combustion unitto be combusted.

FIG. 5 shows the recovery of sulphur by means of the Claus process,wherein several Claus combustion devices are connected sequentially, andthe flow diagram of the post processing of the gases from the Claussystem. The arrangement comprises a first combustion unit having atleast two combustion devices 40, 41 of the Claus system connected inparallel, and condensers 42, 43 connected thereto, and air feedingassemblies 58, 59. The first combustion device of the Claus system isprovided with a feed nozzle 46, which is connected to the source ofconcentrated odorous gases, and with an exhaust nozzle 47, through whichthe oxidized and condensed sulphur compounds of the concentrated odorousgases can be removed from the device as elemental sulphur in a liquid orsolid form. The elemental sulphur can be lead into a collector 44 andfurther to a transporter 45. The combustion device also comprises anexhaust nozzle 48, through which the gaseous, uncondensed sulphurcompounds from the first combustion device of the Claus system can belead into the second combustion device 41 of the Claus system through afeeding nozzle 49. The latter combustion device 41 is provided with anexhaust nozzle 57, through which the oxidized and condensed sulphurcompounds can be removed from the device as elemental sulphur in aliquid or solid form. The elemental sulphur can be lead into thecollector 44 and further to the transporter 45. The second combustionunit of the Claus system is further provided with an exhaust nozzle 50,through which the gaseous, uncondensed sulphur compounds coming thecombustion device of the Claus system can be lead into the secondcombustion unit to be combusted.

As described in connection with FIG. 4, the feed nozzle assembly 46 ofthe first combustion unit can be connected to the distribution piece 51so that part of the concentrated odorous gases can be lead through thedistribution piece and past the first combustion unit directly into thesecond combustion unit to be combusted. Alternatively, the exhaustnozzle 48 of the first Claus combustion device of the Claus unit can beconnected to the distribution piece, through which part of the gasstream coming from the first Claus combustion device can be leaddirectly into the second combustion unit to be combusted.

The exhaust or tail gases, such as sulphur dioxide and TRS, coming fromthe combustion device 41 of the Claus system of the first combustionunit, are lead to be disposed of in at least one combustion device ofthe second combustion unit, such as the odorous gas boiler 53 (which isprovided with a condenser 52), soda recovery boiler 54, lime sludgereburning kiln 55 and/or flame 56.

While the present invention has been illustrated and described withrespect to a particular embodiment thereof, it should be appreciated bythose of ordinary skill in the art that various modifications to thisinvention may be made without departing from the spirit and scope of thepresent invention.

1-25. (canceled)
 26. A method for the desulphurization of thecirculation of chemicals in a sulphate pulp mill, comprising collectingconcentrated odorous gases that contain sulphur compounds and combustingthem to oxidize the sulphur compounds, wherein the concentrated odorousgases are combusted at an air index below one in a first combustionunit, using the Claus process so that at least an essential part of thesulphur compounds are oxidized into elemental sulphur, nonetheless sothat at least 10 molar % of the sulphur compounds taken through theClaus system are taken to be combusted in the combustion device of thesecond combustion unit, and the elemental sulphur is recovered in liquidor solid form.
 27. The method according to claim 26, wherein the airindex of the combustion is about 0.4-0.9.
 28. The method according toclaim 26, wherein the temperature of the combustion is about 1400 to1800° C.
 29. The method according to claim 26, wherein thedesulphurization is integrated into the odorous gas exhaust system ofthe pulp mill.
 30. The method according to claim 26, wherein the sulphuris condensed from the odorous gases after the combustion of the gases.31. The method according to claim 26, wherein the water of the boiler orcooling water circulation of the pulp mill is used to condense thesulphur.
 32. The method according to claim 26, wherein part of thecollected odorous gases are conducted to Claus combustion and part ofthe sulphur compounds from the Claus combustion are conducted toconventional odorous gas combustion, or optionally part of the collectedodorous gases are conducted directly to conventional odorous gascombustion.
 33. The method according to claim 26, wherein at least partof the sulphur compounds of the concentrated odorous gases coming fromthe Claus combustion are conducted to conventional odorous gascombustion.
 34. The method according to claim 26, wherein at least 10molar % of the sulphur compounds from the Claus combustion are conductedto combustion in conventional combustion systems.
 35. The methodaccording to claim 26, wherein part of the concentrated odorous gasesare lead into a first combustion unit, which comprises one or more Claussystem combustion devices and part is lead into the second combustionunit comprising combustion in a soda recovery boiler, odorous gasboiler, lime sludge reburning kiln or flame or into more than one ofthese systems.
 36. The method according to claim 26, wherein at least 10molar %, preferably 30 to 90 molar %, of the collected odorous gases areconducted into one or more Claus system combustion devices forcombustion.
 37. The method according to claim 26, wherein the collectedodorous gases are combusted in the first combustion unit in the Claussystem combustion device and the residual tail gases are conducted tothe second combustion unit to be combusted in the soda recovery boiler,odorous gas boiler, lime sludge reburning kiln or flame or in more thanone of these systems.
 38. The method according to claim 26, wherein 50to 90 molar %, typically 70 to 85 molar %, of the sulphur of the odorousgases lead into the Claus process are combusted to elemental sulphur.39. The method according to claim 26, wherein air, oxygen or a mixturethereof is used in the combustion of odorous gases.
 40. The methodaccording to claim 26, wherein the elemental sulphur obtained from theClaus process is further combusted to sulphur dioxide or processed toform sulphuric acid.
 41. The method according to claim 26, wherein atleast part of the elemental sulphur is fed back into the process, forexample, by adding sulphur to white liquor or feeding the sulphur intothe soda recovery boiler.
 42. The method according to claim 26, whereinthe odorous gases are collected from the source of concentrated odorousgases of a cooking department or an evaporator plant separately or incombination.
 43. The method according to claim 26, wherein the amount ofconcentrated odorous gases is increased by means of a suitable method,such as thermal treatment of black liquor.
 44. A use of the Clausprocess for the desulphurization of the circulation of chemicals in asulphate pulp mill, so that the concentrated odorous gases are combustedwith substoichiometric amounts of air into elemental sulphur in one ormore combustion units of the Claus system and at least 10 molar % of thesulphur compounds taken through the Claus system are taken to becombusted in the combustion device of the at least one second combustionunit.
 45. An arrangement for the desulphurization of the circulation ofchemicals in a sulphate pulp mill, comprising: at least one odorous gascombustion unit, in which the flow of concentrated odorous gases can becollected and into which it can be conducted from a source ofconcentrated odorous gases and wherein the reduced sulphur contained inthe concentrated odorous gases can be combusted, wherein the combustionunit comprises a first combustion unit, which has at least one Claussystem combustion device, which is provided with a feed inlet, which isconnected to the source of concentrated odorous gases, and with anexhaust outlet, through which the oxidized and condensed sulphurcompounds of the concentrated odorous gases can be removed from thedevice as elemental sulphur in a liquid or solid form, and a secondcombustion unit, whereby the first unit has an exhaust nozzle forgaseous, uncondensed sulphur compounds, which is connected to the secondcombustion unit.
 46. The arrangement according to claim 45, wherein thecombustion unit comprises at least two Claus system combustion devicesconnected in series, whereby the first combustion device comprises acombustion assembly for gaseous, uncondensed sulphur compounds, which isconnected to the feed nozzles of the second combustion device.
 47. Thearrangement according to claim 45, wherein the first combustion unitcomprises a Claus system combustion device and a condenser therein. 48.The arrangement according to claim 45, wherein the second combustionunit comprises a odorous gas boiler, soda recovery boiler, lime sludgereburning kiln and/or flame.
 49. The arrangement according to claim 45,wherein the feed nozzle of the first combustion unit is connected to adistribution piece so that part of the concentrated odorous gases can belead through the distribution piece and past the first combustion unitinto the second combustion unit.